1. Field of the Invention
This invention relates to portable machine tools useful for preparing existing openings in heat exchange header structures for butt-welding new heat exchange tubes thereto.
2. Discussion of Related Art
Prior art techniques are known for machining the ends of pipes and tubular conduits in preparation for welding wherein a portable machine tool is secured to the pipe or conduit by means of a centered mandrel locked within the pipe and supporting the machine tool. The machine tool is advanced along the mandrel to feed a cutter head into engagement with the end of the pipe in order to generate a profile suitable for welding the pipe to another pipe or another structure.
Such a technique also has been used to machine openings in structures to which a tube is to be butt-welded. In a procedure of this kind, a mandrel can be locked within the opening and the machine tool can be advanced into engagement with the surface material surrounding the opening on one side of the structure through which the opening extends. During machining, the machine tool is literally supported by the mandrel which must react the torque and bending loads imposed on the cutting head of the machine tool as well as axial and transverse forces imposed during the machining operation.
Large heat exchange systems, such as boilers, condensers, evaporators, etc. utilize multiple heat exchanger tubes that are in direct engagement with a heating or cooling medium and which contain a fluid medium in heat exchange relationship with the heating and cooling medium. Fluid is supplied and extracted from such tubes through larger conduits or tanks located at upper and lower areas of the heat exchanger and which distribute fluid through the heat exchanger tubes.
Because the heat exchanger tubes are exposed to severe environmental conditions, they must be removed and replaced with some frequency. Typically, a defective tube is cut from the header at a location closely adjacent the tube opening in the header and then a machining operation is performed to completely remove the stub end of the tube from the header while preparing the tube opening for butt-welding a replacement tube to the header.
The thickness of the sidewalls of such headers limits the surface area against which the machine tool mandrel may be anchored when it is desired to use a machine tool maneuverable along a mandrel of the type described above. Because of the instability of the mandrel over its full length, high loads imposed on the mandrel result in transverse bending of the mandrel which produces a poor quality finish of the machined surfaces.
It is highly desirable to stabilize the machine tool in a manner to avoid bending of a mandrel supported in a header opening during machining of the opening in preparation for butt-welding a new heat exchange tube to the opening.